Archive for September, 2014

I have an Asus RT-N16 router and after years of perfect service encountered intermittent operation; the unit would operate for hours at a time and then turn off randomly. It would turn itself back on after some time, perhaps 20 minutes. This behavior continued to deteriorate until it would only stay on for perhaps 10 seconds. It felt like an overheating problem, but I was incorrect. As with all problem-solving nowdays, I first consulted the ‘net and found this link.

The capacitor pictured in the link looked perfect, and when I used my ‘scope to examine the quality of the DC power rail it looked fine, but having lots of spare capacitors and nothing to lose I replaced it anyway, and it was fixed!

Moral of the story: capacitors don’t have to be visibly damaged to be faulty. They don’t even have to stop working to need replacement! Also, the quality of the DC power rail is a tricky thing to use, because if the power supply is adequate for most purposes but fails under a particular kind of stress, you will have to a) figure that out and b) observe it while that even is occurring; this can be an elusive thing. It’s possible that had I measured its capacitance, it would have been fine; it could just be that its ESR had risen to unacceptable levels and failed under the high-current demand imposed when the unit’s transmitter quickly used a lot of power. This is a typical failure mode for capacitors; as they get older they still function, but have poor ripple rejection despite having their original value in Farads.

I didn’t have a capacitor of the same physical size, so I removed the defective one, soldered jumper wires and installed a physically large 2500 uF unit where there was room in the chassis a few inches away. It’s been working perfectly for weeks now.